Blasting caps



2 Sheets-Sheet 1 v INVENTOR.

FREDERICK R. SEAVEY Wig- 5T? 0 n N EYS F. R. SEAVEY BLASTING CAPS z 7/! F4 k Jan. 24, 1961 Filed March 7, 1957 F. R. SEAVEY BLASTING CAPS Jan. 24, 1961 2 Sheets-Sheet 2 Filed March 7, 1957 Unite States BLASTING CAPS Filed Mar. 7, 1957, Ser. No. 644,680

2 Claims. (Cl. 86-1) This invention relates to explosive initiators and particularly to detonators of the type employed for initiating the explosion of larger charges of explosives as in military or blasting operations.

Conventional electric detonators consist of an explosive train contain-ed in a cylindrical metal cup having a bridge plug positioned and sealed in the open end thereof. The detonator is generally sealed by pouring a relatively deep layer of a water-proofing composition such as pitch in place while in a heated, fluid condition and subsequently rendered nonfluid by cooling to normal temperature. This pitch seal is normally held in place by pouring a layer of sulfur over it.

Numerous disadvantages are inherent in the use and manufacture of this type of electric detonator and many modifications have been made in electric blasting caps in efiorts to improve their characteristics and particularly their resistance to the entrance of water. The most promising advances made in this direction have been achieved by replacing the metal detonator case with one formed of an organic-plastic material and closing the plastic case by cementing a plastic bridge plug in place to close the open end of the plastic case. Detonators of this type provided with a plastic case and plug are particularly well exemplified in US. Patents 2,415,045 and 2,420,201 granted to Frederick R. Seavey. Such detonators rep-resent a definite advance in the detonator field, but must be manufactured within close tolerances to prevent leakage between the bridge plug and the detonator case.

This particular problem of leakage between the bridge plug and the case was recognized at a very early date, and in an effort to overcome it, electric detonators were provided with a housing of glass. The glass was fused about a pair of lead wires at one end of the tube so as to eliminate the requirement of a bridge plug in the assembly. This attempted solution introduced additional and more serious problems, namely, obtaining a seal between the metal wires and the glass tubing, proper positioning of the lead wires, and the inherent disadvantages of using glass for such a purpose. Furthermore, no provision was made for the inclusion of a bridge wire which is considered essential in present day blasting operations.

Therefore, it is an object of this invention to provide a new and novel blasting cap that overcomes the disadvantages and diificulties of previously known blasting caps. Another object of this invention is to provide an improved process for the manufacture of electric blasting caps. In addition, it is an object of this invention to provide a plastic electric detonator having increased water resistance and a method of making the same.

Other objects will become apparent to those skilled in the art when the following description is read in connection with the accompanying drawings in which:

Figure 1 is a longitudinal view partially in section of an assembled electric blasting cap constructed in accordance with the present invention;

Figure 2 is a longitudinal sectional view of a detonator component used in carrying out this invention;

" aten Patented Jan. 24, 1961 Figure 3 is a longitudinal sectional view of apparatus that can be used in carrying out the process of the present invention;

Figure 4 is a longitudinal view partially in section of an electric detonator rep-resenting another embodiment of this invention; and

Figure 5 is a sectional view taken along the line 55 of Figure 4.

In accordance with the present invention, electric detonators having an integral bridge plug and shell wall are formed by providing a pair of plastic coated lead wires with a positioning means adjacent their bared ends, extending a bridge wire between the bared ends of the lead wires, molding an integral bridge plug and case about the spaced lead wires and sealing the open end of the case remote from the bridge wire assembly. The bottom of the detonator case is sealed, after the explosive train has been introduced thereinto, by cementing or otherwise securing a water-impermeable disc to the bottom of the detonator.

While it is essential that the lead wires be provided with positioning means adjacent their bared ends, such positioning means can be present throughout the length of the insulated lead wires. Thus, the lead wire positioning means can be attached to the bared lead wires solely in proximity to the bridge wire; or alternatively, webbed duplex insulated lead wires can be used.

Referring to Figure l of the drawings for an illustrative embodiment of the invention, a thermoplastic bridge plug 1 carrying lead wires 2 is provided with an integral skirt 3 forming the sidewall of the detonator case. The lead wires are embedded in the bridge plug 1 and also in cylindrical thermoplastic positioning means 4, and bridge wire 5 is attached to the ends of the lead wires. The initiator can contain any suitable explosive train or deflagrating composition and in this preferred embodiment, the case is loaded with an ignition composition 6 surrounding the bridge wire, a suitable initiating charge 7, and a base charge 8. In this preferred embodiment, the base charge is formed of pre-pelleted explosive material, but it will be readily appreciated that the explosive charge can be compressed in situ if so desired. The internal surface of the shell wall is provided with a shoulder 9 to accommodate flange 10 of a thermoplastic disc 11 sealing the base of the detonator. The flange of the closure disc has a diameter closely approximating that of the internal diameter of the shell wall below the shoulder 9 and is sealed in place by means of a suitable cement or compatible solvent.

In this preferred embodiment, the lead wire insulation, the integral bridge plug and shell wall, the bridge wire positioning means, and the closure for the base of the case are all formed of plasticized polyvinyl chloride and the disc closing the bottom of the detonator is sealed in place by applying a coating of tetrahydrofuran to the external surface of its flange before inserting it into place. While polyvinyl chloride was utilized in this preferred embodiment, practically all of the commercially available thermoplastic materials can be used in the formulation of the structural components. Thus, in accordance with this invention, they can be formed of ethylcellulose, polyvinyl chloride, polyvinyl acetate, polyvinyl chlorideacetate copolymers, polyamides, polystyrene, polystyreneacronitrile polymers, and the like. The lead Wire insulation can be prepared from the above thermoplastic materials as well as from neoprene, polyethylene, and synthetic modified or halogenated rubbers.

Figure 2 illustrates the preassembly of the detonator lead wires 2, the bridge wire 5 and the lead wire positioning means 4. As shown in this figure, the lead wires 2 pass completely through cylindrical lead wire positioning means '4 in spaced parallel relationship and carry bridge wire 5. The lead wires are held'in this relationship by any conventional means while the positioning means is molded about them and their bared ends are then con nected with bridge wire 5. After the assembly ot Figure 2 has beencompleted, itis inserted in the molding apparatus as shown in Figure 3 of the drawing.

As shown in Figure 3, theassembly of Figure 2 is positioned in a mold cavity 12 by its lead wires 2 which are maintained in a stationary position by being snugly positioned between the upper mold portions 13 and 14. Furthermore, the proper positioning of the assembly in the mold cavity is insured by heating lead wire support 4 in the cylindrical cavity 16 in the top of the cylindricalv steel insert 15. Mold insert is also provided with a second smaller cavity 17 at the baseof cavity 16 to accommodate the ends of the lead wires and the attached bridge wire. Cavity lfi'conforms closely to the configuration of bridge support 4 and is so proportioned thatthe bridge support forms a tight frictionalfit'with the walls of thecavity. Such a lit is essential in order to prevent ingress of the molding material into cavity .17 surrounding the bridge wire.

In operation, the bridge support is fitted into cavity 16 and mold parts 13 and 14 are'assembled together with V the insert 15 so as to form mold cavity 12 containing a "portion of bridge support 4 and a segment of the lead wires 2. Thernold eiements are held in the relationship The web 13 serves to position the lead wires and to maintairt them in the proper spaced relation as they pass through the bridge plug 1 of the detonator.

tions and variations will present themselves to those skilled in the art without departing from the spirit of the invention, and accordingly it is to be distinctly understood that the invention is not limited'to the details of the foregoing disclosures save as indicated by the appended claims.

Having thus described the invention, what is claimed and desired tobesecured by Letters Patent is: p p

' 1. A method ofv manufacturing electric detonators which comprises molding a cylindrical plastic'body about a pair of spacedplastic insulatedlead wires adjacent the bared ends thereof,v extending a bridge wire between the bated ends of the leadwires, one end of the cylindrical shown in the drawing by any conventional means which do not form a part of the present invention and, thus, are

not shown. After theapparatus had been assembled, a

polyvinyl chloride molding composition'was introducedv through gate 18 at a temperature of about 250 F. Although only one gateis shown, it will be readily appreciated that a plurality of gates strategically positioned can be used. After the mold cavity was completely filled with the molding composition, the: composition Was permitted to solidify, the molding apparatus dissembled, and

the unitary plastic bridge plug and vdetonator sidewall I removed from the'mold. 1

In this particular embodiment, a polyvinyl chloride molding composition was used, and the lead wire insulation, as well as the bridge support was formed of the same material. While the utilization of the same thermoplastic material throughout the assembly is preferred and eliminates a number of problems, it will be readily appreciated that each of the thermoplastic components can readily be made from different materials, the only requirement being that they have similar thermal expansion coefficients and are adaptable to scaling with suitable solvents or cements.

Another embodiment illustrating a detonator prepared in accordance with this invention is set forth in Figures 4 and 5 of the drawing. The detonator of this embodiment is also provided with a thermoplastic bridge plug 1 provided with an integral skirt 3 forming the sidewall of the detonator. A suitable explosive train is contained within the detonator and identified by numerals 6, 7 and 8. As described above, the base of the detonator is sealed by a closure disc 11 suitably anchored to the sidewall of the detonator case. In this particular embodiment, lead wires 12 are connected together throughout the major portion of their length by a plastic web 13.

body facing the bridge; wire, molding a one-piece combinationthermoplastic bridge plug and case about the side wall and the other end of the cylindrical plastic body l and also abouta portion of the lead wires-adjacent said otherend of the plastic body, the case being openat its 7 end opposite the lead wires, introducing an explosive,

material through the open endof the case, and providing a bottom closure for the case.

2.'A process for the manufacture of electric detona:

tors whichcornprises providing a pair of plastic insulated 1 lead wires. having bared terminal portions, molding a positioning means about the wires adjacent thetermi- 'nalportions thereof, connecting the terminal portions 7 with a bridge wire, covering the bridge wire and the terminal portions of the lead wires, molding a plastic bridge plug aboutthe positioningmeans and a portion of the lead wires above the positioning means, the bridge plug being provided with a substantially cylindrical skirt portion about the bridge wire and extending axially so as to form a one-piece combination plastic plug and openended case, uncovering the bridge wire and the terminal bared portion of the lead wires, introducing an explosive charge through the open end of the case, and providing a bottom closure for the case.

References Cited in the file of this patent UNITED STATES PATENTS 720,073 Schmitt et a1 Feb. 10, 1903 2,423,837 Martin July 15, 1947 2,722,155 Pauls Nov. 1, 1955 2,750,831 Long June 19, 1956 2,763,211 Blackburn Sept. 18, 1956 2,772,633 Barocca Dec. 4, 1956 2,777,389 Lawrence a- Jan. 15, 1957 2,801,585 Smith Aug. 6, 1957 2,802,421 Horne et 'al. Aug. 13, 1957 Thus, when Patent No. 2,968,985 January 24, I 1961 UNITED :STATES- PATENTOFFICE CERTIFICATE OF CORRECTION Frederick R. Seavey It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line I2 for "heating" read seating Signed and sealed this 2nd day of January 1962 (SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer I Commissioner of Patents 

